Don't compute ((uint64_t)1)<<64 in round_to_power_of_2

This would be undefined behavior if it happened. (It can't actually
happen as we're using round_to_power_of_2, since we would have to
be trying to allocate exabytes of data.)

While we're at it, fix the behavior of round_to_power_of_2(0),
and document the function better.

Fix for bug 6831.

changes/bug6831

@@ -0,0 +1,4 @@
+  o Minor bugfixes:
+    - Fix round_to_power_of_2 so it doesn't invoke undefined behavior
+      with large values. This was untriggered, but nevertheless incorrect.
+      Fixes bug 6831; bugfix on 0.2.0.1-alpha.

src/common/util.c

@@ -394,12 +394,24 @@ tor_log2(uint64_t u64)
   return r;
 }
 
-/** Return the power of 2 closest to <b>u64</b>. */
+/** Return the power of 2 in range [1,UINT64_MAX] closest to <b>u64</b>.  If
+ * there are two powers of 2 equally close, round down. */
 uint64_t
 round_to_power_of_2(uint64_t u64)
 {
-  int lg2 = tor_log2(u64);
-  uint64_t low = U64_LITERAL(1) << lg2, high = U64_LITERAL(1) << (lg2+1);
+  int lg2;
+  uint64_t low;
+  uint64_t high;
+  if (u64 == 0)
+    return 1;
+
+  lg2 = tor_log2(u64);
+  low = U64_LITERAL(1) << lg2;
+
+  if (lg2 == 63)
+    return low;
+
+  high = U64_LITERAL(1) << (lg2+1);
   if (high - u64 < u64 - low)
     return high;
   else

src/test/test_util.c

@@ -1109,6 +1109,7 @@ test_util_pow2(void)
   test_eq(tor_log2(64), 6);
   test_eq(tor_log2(65), 6);
   test_eq(tor_log2(63), 5);
+  test_eq(tor_log2(0), 0); /* incorrect mathematically, but as specified */
   test_eq(tor_log2(1), 0);
   test_eq(tor_log2(2), 1);
   test_eq(tor_log2(3), 1);
@@ -1123,7 +1124,17 @@ test_util_pow2(void)
   test_eq(round_to_power_of_2(130), 128);
   test_eq(round_to_power_of_2(U64_LITERAL(40000000000000000)),
           U64_LITERAL(1)<<55);
-  test_eq(round_to_power_of_2(0), 2);
+  test_eq(round_to_power_of_2(U64_LITERAL(0xffffffffffffffff)),
+          U64_LITERAL(1)<<63);
+  test_eq(round_to_power_of_2(0), 1);
+  test_eq(round_to_power_of_2(1), 1);
+  test_eq(round_to_power_of_2(2), 2);
+  test_eq(round_to_power_of_2(3), 2);
+  test_eq(round_to_power_of_2(4), 4);
+  test_eq(round_to_power_of_2(4), 4);
+  test_eq(round_to_power_of_2(5), 4);
+  test_eq(round_to_power_of_2(6), 4);
+  test_eq(round_to_power_of_2(7), 8);
 
  done:
   ;